Operational conventional purification processes such as distillation, rectification and adsorption can be economically carried out only with large amounts of gas. If one uses such purification processes with small amounts of gas and if one sets extremely high requirements for purity, the purified product gases become unusually expensive. This is the case with the production and purification of doping gases for the semi-conductor industry. These conditions are even more complex when the gases to be purified are highly toxic, self igniting or unstable.
Examples of such gases are silane, disilane, dichlorosilane, germane, borane, arsine, hydrogen chloride, ammonia, tetrafluoromethane and various metallic organic compounds. Among the contaminating components are: nitrogen, oxygen, inert gases, hydrocarbons, carbon dioxide and carbon monoxide. These impurities are, as a rule, more easily volatilized than the gases to be purified.